Missing documentation for public C-API function

[wrongnull]

Documentation

There are no documentation for the PyCode_GetFirstFree and incorrect return type for PyCode_GetNumFree

Linked PRs

• gh-115653: Add docs for the PyCode_GetFirstFree and correct return type for the PyCode_GetNumFree #115654
• [3.12] gh-115653: Document PyCode_GetFirstFree() (GH-115654) #115752

[erlend-aasland]

Backport PR is approved and queued for merge. Closing.